Circular knitting machines have rotating cylinders and knitting instrumentalities which produce a tubular fabric which rotates with the cylinder. It is known to withdraw the fabric being formed downwardly through the rotating cylinder by a fabric take-up mechanism which flattens the tubular fabric and winds the flattened fabric around a take-up roll. In such circular knitting machines, the take-up mechanism rotates synchronously with the cylinder to avoid twisting of the fabric as it rotates with the needle cylinder.
Conventionally, the take-up speed of the take-up mechanism of the circular knitting machine is adjusted while the machine is not in operation to a preset value calculated to maintain the fabric under tension when the knitting machine knits the maximum length of fabric it is capable of producing for that type of fabric. However, if the length of fabric being knitted varies from this maximum length because of different stitch construction, different types of yarn, or variation in the stitch lengths being formed, tension in the fabric will gradually increase during knitting to the point that the fabric is torn or knitting needles are placed under sufficient stress that breakage occurs. Previous attempts to solve this problem have either failed or have been only partially successful.
One such prior attempt provides a detector for detecting substantial increases in tension in the fabric and a stop motion to stop the knitting machine when such tension exceeds a predetermined maximum allowable value. While avoiding torn fabric or breakage of needles, such an attempt resulted in substantial downtime for knitting machines and reduced production, as well as substantial operator time in making manual adjustments of the take-up speed of the knitting machines.
Another suggested solution to the problem disclosed in the U.S. Pat. No. 4,671,083, owned by the same assignee as is this application. In this patent, a variable speed drive is disclosed involving a belt and variable pulleys which respond somewhat to the tension in the fabric to vary the speed of the take-up mechanism to reduce the tension in the fabric due to belt slippage in the pulleys. The drive mechanism for the take-up unit disclosed in U.S. Pat. No. 4,671,083 has been partially successful in addressing the problem but still requires substantial operator adjustment of the drive mechanism and only partially responds to increases and decreases in tension in the knitted fabric.